This page is meant to explain exactly how gas gets from your tank to your rig. For the most part, we all have a functional understanding of what knobs we can turn to change the pressure at our rig. However, with a stronger understanding of how gas actually flows from tank to rig, hopefully this page allows you to:
a) hook things up properly
b) troubleshoot when something feels awry
c) minimize wasteful gas useage!
Below is a thorough explanation of the flow in the 2p room, but the exact same principles apply to the gas flow in the main lab.
General Flow:
The source of the air in the 2p room is in the gas closet just outside. Two tanks are hooked up and are the source of all of the air. At the start of the day, those tanks should be turned on. Both of them!
Air then enters a main pressure regulator (called just “regulator” throughout). This big regulator is fancy because it can keep one input line (from the tanks) closed while the other is still pumping air. The priority tank is defined by the “Primary” lever. Once there is no air flowing through that line, then the regulator opens to the secondary tank. There are two measurements read out by this regulator: one meter closer to the gas tank tells you what the pressure coming from the tank is (one meter per gas tank line), and the bigger meter tells you want the pressure leaving the regulator is. This is what the regulator is regulating, so turning the knob on the regulator changes this pressure. This is the pressure that enters the 2p room, and is thus the pressure felt by any given rig.
In the 2p room, air is then split 4 times, once per 2p rig. Each rig (SHOULD BE, BUT IS NOT YET) equipped with the same 3 components that ultimately control the flow of air supporting the ball.
- A valve. This allows air to flow to the rig. It’s located just where the air lines come out of the ceiling, above each rig. If you are not running an experiment, this should be CLOSED. Otherwise, air is getting pumped through your set up and that is wasteful. This should be touched only at the start and end of your experiment, to open and close the valve.
- A regulator. This isolates the pressure felt at your rig from the pressure entering the room. This should be located just past the valve, close to the ceiling. This is important because regulators take non-zero time to equilibrate the outgoing pressure when the inlet or outlet flow changes - this is because it takes non-zero time to build up the air pressure within the regulator. More regulators in series generate a more constant apparent air pressure, because the adjustment made by each individual regulator is less. Specifically, if the pressure at each rig is set only by the main regulator, then opening and closing valves at each rig (changing the total flux) will have non-negligible (albeit transient) changes to the air pressure feeding the rigs, since this pressure is only controlled by one regulator. Having a regulator at each rig serves to isolate them, and maintains constant pressure during an experiment.
- A flow meter. This is used for fine control of the air flow that ultimately reaches the ball. It is important to note that this controls how much gas actually passes through the flow meter - it does not change the pressure! If pressure is voltage (set by your battery, or regulator), then the flow meter is a resistor, which changes the amount of current through the circuit. More current = more air to the ball. Importantly, flow meters accept gas within a certain pressure range.
Lastly, it’s important to know that regulators have a minimum and maximum pressure that they can accept. To make everyone’s life easier, we really only want to turn knobs AFTER the regulators, if we can help it. In the 2p room, this means that we set the pressure on the main regulator to some value in the acceptable range of the individual regulators, and then we set the individual regulators in the acceptable range of the flow meters. And then we NEVER TOUCH THE REGULATORS, but we do touch the flow meters, for fine control of the air flow at our ball.
Air and carbogen in the main lab work much the same way: anywhere that we have a tank farm, we have a regulator hooked up right there at the source. Often these regulators are fancy ones where you can set a primary and a backup tank, and both should be opened (though we keep the lever facing the primary until it is fully depleted). You can then follow the gas lines to individual rigs, or to dissection stations, and see either more regulators or flow meters.
CO2 is not quite as involved, but there is a regulator attached directly to the tank. You can turn this knob to change the pressure (and flow rate) of CO2 leaving your needle (or, if you’re child of Satan, gun). There usually are not two regulators in series here, because constant air flow is not quite as important and these lines don’t split as many times.
Hopefully you now know how the system works!
General rules of thumb:
- To change the flow rate, fiddle the downstream-most knob first. If there are many knobs in series, double check with somebody before touching an upstream knob, as that may effect other people.
- To minimize gas waste, make sure that valves to unused gas lines are closed! We will not waste any gas if only gas lines in use have open valves!
General FAQ:
How do I change the pressure felt by my ball?
You should really only need to fiddle with the knob on YOUR PERSONAL FLOW METER to control the air pressure at your rig. Do not touch other knobs. If you feel that you cannot get the right pressure at your rig, check in with the gas czar, because it means we’ll have to fiddle with other knobs that will affect everybody else.
Okay, so what if my rig doesn’t have a personal regulator after the valve…
This isn’t a cardinal sin (in fact, most people haven’t had it this way), but it does mean that the pressure at your rig is not isolated, and is thus subject to the pressure coming from the main regulator. If you’re the only person running an experiment at the time, it doens’t actually matter. But it’s a good idea because, in the worst case, this means that your ball may fluctuate mid experiment when somebody opens their valve. In the more moderate case, it’s a good failsafe because it means the pressure at your rig is not dictated by the pressure of the main regulator, in the event that somebody tweaks that. Remember, the pressure from the main regulator is set to be higher that that desired by any ball set up (so that it can be regulated by individual rigs). Your life will be easier if you just regulate, and maintain constant, the pressure that enter your rig, such that it doesn’t depend on who else has their valve open, and we can just keep those (5 total) regulators fixed.
What is the horrible hissing noise?
It could be a leak somewhere. More likely, though, it’s this: Technically, air flowing is always making some hissing noise, the same way that you can hear water passing through pipes when you turn a faucet on. At normal flow rates, this hissing isn’t audible, though. The horrible hissing noise means that MORE air than usual is passing through the pipes.
Regulators change the flow rate to maintain a certain air pressure. Our main regulators (the ones directly modulating the pressure from the tanks) are set to a high pressure, because they get split into a number of lines (individual rigs). This means that if many valves are open AFTER the regulator, a TON of air is getting spewed through the regulator, and that super high air flow is making a sound in the pipes. Electrically, it’s like dropping the resistance - for a fixed voltage (pressure), you’ll super increase the current in the circuit. This is what a short circuit is, basically. This can be fixed by following the gas lines, and making sure that there is no leak AFTER the regulator (maybe someone’s tubes got disconnected, causing a low resistance, and making it so that more air than usual is being passed through the regulator, causing more hissing than usual).